Cast stone is a product that simulates natural cut stone used as an architectural feature, trim, ornament, or facing of buildings, walls, or other structures. Cast stone is made from a cementitious material, such as concrete, that includes fine and course aggregates, Portland cement, water, coloring, and optional chemical admixtures. Two methods to manufacture cast stone are the vibrant dry tamp method and the wet cast method.
Each method requires design mixes that must be carefully proportioned to form products having maximum density and texture resembling natural stone. White Portland cement is usually used to achieve lighter colors and color consistency. However, blending of grey Portland cement and coloring pigments together with white cement in order to achieve color is also common practice. Since the cement to aggregate ratio of 1:3 is normally used, a properly cured cast stone unit will have a high compressive strength of approximately 6,500 psi and a lower cold water absorption rate of approximately 6%, both of which are better than natural limestone or normal concrete. The vibrant dry tamp method, due to its inherent lower water-cement ratio typically yields higher compressive strengths than low slump concrete.
The vibrant dry tamp method typically involves the vibratory ramming of concrete against a rigid formwork until it is densely compacted and ready for immediate removal from the mold. This process enables as many as 100 pieces to be cast from a single mold in an eight hour day. However, the vibrant dry tamp method generally requires one flat, unexposed side to the stone design, thereby making complex multi-faced shapes, such as an “L” shape and similar shapes more costly to produce.
The wet cast process for manufacturing cast stone is similar to the manufacturing process used for making precast concrete, but produces a finish much more closely resembling natural stone. Mix designs in the wet cast process usually require graded sieve coarse aggregates typically one-half inch or smaller and are composed of an abundance of fine aggregates, which when combined with careful manual and vibratory concrete placement techniques, leaves little or no voids after finishing of exposed surfaces. The compression strength is approximately 6,500 psi with a lower cold water absorption factor than natural stone, making cast stone made from the wet cast process less permeable to water, weather, and dirt.
In some processes, surface finishing is employed to remove the cement “skin” from the outer surface of the stone. This removal exposes the fine aggregates and assures that the product will undergo minimal color and texture changes as a result of weathering. Muriatic (hydrochloric) acid etching is the most popular method of finishing cast stone because of the brilliance of the etched aggregates and the ability of the resulting finished surface to remain clean. A honed or polished finish which provides a glossy finish similar to granite or marble may also be employed. However, these processes for manufacturing cast stone are costly and labor intensive.
Therefore, there is a need in the art for a system and method of manufacturing cast stone where human intervention is minimized, thereby minimizing manufacturing costs and risk of injury.